Is air present only in atmosphere?…No..! Air is also present in soil which is known as soil air
What is soil composed of?
Soil is composed of solid, liquid and gaseous constituents in various proportions.
Also read our various other related articles under Soil science Category
What are Pore spaces?
It is the part of the soil, not occupied by soil solids.
The size and arrangement of soil particles or soil aggregates will determine the pore space.
This pore space is normally occupied by soil water and soil air in reciprocally varying amounts.
An important characteristic of the pore space is its continuity.
The degree of continuity of both soil water and its air is of great importance in determining the physical properties of soil.
Watch this video for important points of study
Composition of Soil air
The composition of soil air is not the same as that of the atmosphere
The plant life and microorganism cause the soil atmosphere to become dynamic with respect to the ratio of oxygen to carbon-di-oxide.
The principal components of soil air are nitrogen, oxygen, inert gases, carbondioxide, water vapour and hydrogen. Methane, hydrogen sulfide etc., are present in negligible quantities.
The proportion of nitrogen plus argon is usually found to be equal to that in the atmosphere and is 79 per cent by volume.
Oxygen and carbondioxide vary in complementary proportions to make up the remaining 21 per cent.
The following information shows the names of Gases, Soil air (%) and Atmospheric air (%)
Nitrogen – 79.20% in Soil air and 79.00 % in atmospheric air
Oxygen – 20.60 % in sool air and 20.97 % in atmospheric air
Carbondioxide – 0.50 % (it may change depending on various factors) in soil air and 0.033 % in atmospheric air
CO2 content varkes in soil from 10-10000 times to that of atmospheric air
Since soil air is located in the net work of soil pores separated by soil solids,
it varies in composition from place to place in the soil. In local pockets, reactions involving gases can greatly modify the composition of the soil air.
The composition of soil air at the surface and
atmospheric air are mostly similar, because of rapid gas exchange at surface, but this similarity decreases with increase in the depth of soil.
The composition of air is the resultant of two sets of processes that function simultaneously viz : chemical and biological reactions that result in the production of CO2 at the use of O2; and physical processes that affect the interchange of gaseous constituents between
the soil voids and the atmosphere.
First process accentuates the differences, whereas the second one minimizes the differences.
How much minimum concentration of oxygen can a soil have?
Oxygen concentration in the soil air should be at least 10%.
Gaseous exchange in soil
The exchange of gases in between soil air and atmospheric air is referred to as soil aeration. This is very important for growth of plants and soil microorganisms and thin biological
Two mechanisms have been identified in the interchange of gases between soil voids and the atmosphere i.e. mass flow and diffusive flow.
Mass flow of gases in to and out of the soil occurs whenever there is a difference in total pressure between the soil air and atmosphere. This difference is mainly due to expansion and contraction of soil gases due to changes in temperature and barometric pressure, air replacement through rainfall, irrigation and drainage.
Diffusion is the random movement of molecules (molecular motion) of a gas or
of a liquid.
Net movement of gases by diffusion occurs when the partial pressures of individual
gases in two neighbouring systems are different, but the total pressure is the same in both.
Diffusion accounts for >90% of gaseous exchange.
In soils, due to biological activity, there is a continuous production of CO2 at the
exhaustion of oxygen.
This results in the increased partial pressure of CO2 and reduced partial pressure of oxygen.
It leads to continuous interchange of CO2 and O2 in between soil and atmosphere.
According to Fick’s law, diffusion is a function of the concentration gradient, the diffusion co-efficient of the medium, and the cross sectional area.
dQ = DA (dc/dx)dt
where dQ is the mass flow (moles) during the time at across area A (sq.cm), dc/dx the
concentration gradient [moles/cc (cm) ], and D the proportionally constant or diffusion
Characterization of soil aeration
There are various parameters that can be used for characterizing soil aeration like
The volume percentage of soil air
It Indicates the part of pore space filled with air. It should be minimum of 10-12 % is required.
The oxygen diffusion rate: Determines the rate at which oxygen can be replenished. if it is used by respiring plant roots or microbes.
Oxidation reduction potential
Indicates whether the soil is in oxidized or reduced state.
It is a measure of tendency of a system to reduce or oxidize expressed as negative and positive potentials, respectively.
Effect of soil air on plant growth, soil properties and nutrient availability
Soil air with its constituents plays a vital role in modifying the physical, chemical and biological properties of the soils, there by governs the nutrient and water availability and ultimately the plant growth.
Under poor aerated conditions development of plant roots will be restricted or inhibited.
Absorption of water and nutrients will be decreased.
Toxic substances will be formed
Soil aeration influences the activity of soil micro organism and also affects the rate of decomposition of organic matter.
BNF and many nutrient transformations will be carried out by microbes under will aerated conditions.
Many nutrients like Fe and Mn will be at their toxic levels under poorly aerated conditions.
Many toxic organic acids like lactic, butyric and citric acid etc., will be found under anaerobic conditions and cause injury to plant roots thereby minimize the ability of roots to absorb nutrients and water.
Zn and Mn availability will be reduced under poor aeration conditions.
Incidence and virulence of diseases largely controlled by soil aeration.
Measures to improve soil aeration
Since diffusion is the main agent for air renewal, a more open structure, a lower moisture content and a higher temperature will be helpful in increasing diffusion rate of air.
Tillage, incorporation of organic matter and mulching are the standard methods to
create porosity in the soil system.
Providing drainage facilities is a remedy to improve air capacity of soil.
Further reading on – comprehensive overview
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Can you answer this MCQ?
The percentage of oxygen in the soil air should not be less than?
A. 20 %
B. 30 %
C. 10 %
D. 1 %
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